Iron, aluminum, copper and silicon seldom occur in nature as elementary substances and mostly occur as compounds such as oxides. Accordingly, when these are used as a structural material, electroconductive material or semiconductor material, these oxides should be reduced in order to remove impurities. However, impurities cannot be sufficiently removed by mere reduction of these oxides. Accordingly, the amount of impurities contained therein should be further reduced. A step of reducing the amount of such impurities is referred to as purification.
In purification of iron used as a structural material, for example, pig iron removed from a blast furnace is contacted with a molten oxide called slag, whereby impurities such as sulfur and phosphorus remarkably deteriorating toughness are incorporated into the slag thereby reducing the content of impurities in the pig iron. With respect to carbon as an impurity capable of determining the mechanical strength of steel, an oxygen gas is blown into pig iron, whereby carbon in the pig iron is oxidized and removed as carbon dioxide gas, thereby regulating the amount of carbon in the pig iron.
In purification of copper used as an electroconductive material, the phenomenon in which the segregation coefficient of impurities, that is, the ratio of the concentration of impurities in solid copper to the concentration of impurities in molten copper in an equilibrium state, is low can be utilized to lower the concentration of impurities in solid copper by solidifying molten copper at such a low rate as to be in an almost equilibrium state.
In purification of silicon used as a semiconductor material, silicon with a purity of 98% or more obtained by reducing silica is converted into gases such as silane (SiH4) and trichlorosilane (SiHCl3), and these gases are decomposed in a Belljar furnace or reduced with hydrogen, whereby polycrystalline silicon with a purity of about 11 N can be obtained. This polycrystalline silicon is used for growth of single-crystalline silicon, whereby single-crystalline silicon used in production of electronic devices such as LSI can be obtained. For obtaining single-crystalline silicon used in production of electronic devices, a very complicated manufacturing process and strict management of the manufacturing process are necessary, thus inevitably increasing the production costs thereof.
On one hand, demand for solar cell is rapidly increasing in recent years, owing to increasing awareness of energy problems such as depletion of fossil-fuel resources and environmental problems such as global warming. The required purity of silicon used in production of solar cells is about 6 N. Accordingly, irregular products of silicon for electronic devices, which have been previously used in production of solar cells, have excess qualities as silicon for solar cells.
Because the amount of generated irregular products of silicon for electronic devices has been surpassing the demanded amount of solar cells up to now, there has been no problem. However, the demanded amount of solar cells will certainly surpass the amount of generated irregular products of silicon for electronic devices in the future, so establishment of techniques for inexpensively manufacturing silicon for solar cells is strongly demanded. As such techniques, techniques of purification by a metallurgical method utilizing the redox reaction or solidification segregation described above attract attention in recent years.
Among impurities contained in silicon for solar cells, phosphorus and boron both have a high segregation coefficient. Accordingly, the method of purification by solidification segregation is known to have little effect for removal of phosphorus and boron.
It follows that with respect to removal of phosphorus, Japanese Patent Laying-Open No. 6-227808 (Patent Document 1) discloses a method for releasing phosphorus into a gaseous phase by keeping molten silicon in a reduced-pressure atmosphere.
With respect to removal of boron, Japanese Patent Laying-Open No. 4-228414 (Patent Document 2) discloses a method that involves irradiating the surface of molten silicon with plasma of a mixed gas containing an inert gas and water vapor. U.S. Pat. No. 5,972,107 (Patent Document 3) discloses a method that involves dipping a torch of burning hydrogen and oxygen into molten silicon. Japanese Patent Laying-Open No. 2001-58811 (Patent Document 4) discloses a method that involves blowing a treating gas into molten silicon under stirring. Japanese Patent Laying-Open No. 8-73209 (Patent Document 5) discloses a method that involves continuously introducing slag into molten silicon. All of these methods for removing boron are those for removing boron oxides from molten silicon.
Patent Document 1: Japanese Patent Laying-Open No. 6-227808
Patent Document 2: Japanese Patent Laying-Open No. 4-228414
Patent Document 3: U.S. Pat. No. 5,972,107
Patent Document 4: Japanese Patent Laying-Open No. 2001-58811
Patent Document 5: Japanese Patent Laying-Open No. 8-73209